The long-range goal of this project is to lay the groundwork for investigations of the clinical significance of dihydropteridine reductase (DHPR) to monoamine-related neurological and psychiatric disorders. This enzyme, which provides an essential cofactor (tetrahydrobiopterin, or BH4) for the hydroxylation of aromatic amino acids in monoamine synthesis, is known to be required for normal brain function. Deficiency in DHPR or a defect in BH4 synthesis causes a variant form of hyperphenylalaninemia. Reduced levels of BH4 in cerebrospinal fluids of patients with Parkinson's disease, torsion dystonia, Alzheimer's disease, Steel-Richardson syndrome, and Huntington's chorea have also been reported. Clinical studies of the enzyme have been hampered, however, by a lack of information about the regulatory control of its activity, especially its structural and functional relationships with its endogenous and exogenous inhibitors. Progress in future clinical studies will depend on basic investigations of the role of DHPR in tyrosine hydroxylation and the regulation of monoamine synthesis in the central nervous system. We propose to begin our studies with a comparison of human brain DHPR with the multiple forms of the enzyme found in human platelets and liver. Then we will investigate the regulation of DHPR activity by pharmacological agents and endogenous factors in rats, an in vitro model system using synaptosomes from rat brain striatum. These experimental systems will allow selective inhibition studies to determine how inhibition of either tyrosine hydroxylase or DHPR activity affects the hydroxylation system and catecholamine synthesis. If we find that brain and platelet DHPR are identical, we will be able at a later time to use platelet samples for screening for inactive or variant forms of the enzyme in the human population, including patients with neurological and psychiatric disorders.